Breakage of glass begins from the surface when the tensile stress arising on the surface of the glass because of the external force exceeds the tensile strength of the glass, except for unusual cases. The durability of the glass against the tensile stress is strongly affected by minute flows known as the “Griffith flaws” which exist on the surface of the glass. Therefore, in order to improve the durability, it is effective to provide a compressive layer to the glass surface so as to decrease the tensile stress arising from the external force, thereby preventing progress of the flaws. The compressive layer is formed by chemical reinforcements or physical reinforcements.
According to the physical reinforcements, the compressive layer is formed on the surface of glass in such a manner to cool the glass with high temperature to the room temperature at a high rate so as to stress the glass in the direction of the thickness thereof residually. As the physical reinforcements, air blast coolings are most widely put to practical use.
According to the air blast coolings, the glass is heated to around the softening temperature, and then the surface of the glass is cooled with compressed-air blast at a high rate to form the compressive layer on the surface and to form the tensile layer within the glass. As a result, the strength of the glass is improved.
It is known that the surface residual stress of the glass that arises from the air blast coolings depends on the temperature difference between the surface and the inner portion of the glass during the application of cooling. As the most simple approximation, when the glass with high temperature is cooled at a high rate, if the heat release Q is assumed to be constant, the maximum temperature difference between the surface and the inner portion of the glass (Δθ)max is approximately determined from the following equation:(Δθmax)=tQ/8kwherein t is a thickness of the glass [m], Q is a heat release [W/m2], k is a thermal conductivity [W/m·K].
Assuming that the time required to relax the strain is sufficiently short and the temperature gradient does not vary during the application of cooling, the surface compressive stress F of the glass with the ordinary room temperature is determined from the following equation:F=α·E/(1−σ)·2/3·(ΔQ)max
wherein α is a mean linear expansion coefficient, E is Young's modulus, and σ is Poisson's ratio. The equation shows that the compressive stress F is increased by the increase of α and E.
Conventionally, the float glass plate employed for the window glass of the vehicle has a thickness from 3.5 to 4.8 mm. Recently, there has been strong demand to reduce the thickness of the window glass so as to decrease fuel cost by lightening the vehicle. However, assuming the area of the glass is the same, the glass is reduced in the heat capacity as the glass becomes thin, thus decreasing (ΔQ)max. As a result, the surface compressive stress F decreases. Therefore, some reinforced glasses are proposed so as to compensate for that.
A reinforced glass produced in accordance with a method for producing a reinforced glass disclosed in Japanese Patent (JP) publication H6-53592B is practically composed of (weight percent):    SiO2:63-75%;    Al2O3:1.5-7%;    TiO2:0-6%;    Al2O3+TiO2:3-7%;    MgO:0-10%;    CaO:5-15%;    MgO+CaO:6-20%;    Na2O:8-18%;    K2O:0-5%; and    Na2O+K2O:10-20%,and the glass has a liquidus temperature of equal to or less than 1150° C.
A glass to be reinforced easily disclosed in JP H4-60059B has the following composition (weight percent):    SiO2:68-71%;    Al2O3:1.6-3.0%;    MgO:2.0-4.0%;    CaO:8.5-11.0%;    Na2O:12.5-16.0%;    K2O:0.9-3.0%,wherein the total amount of these ingredients is equal to or more than 97%,    SiO2+Al2O3:70.0-73.0%;    MgO+CaO:12.0-15.0%; and    Na2O+K2O:13.5-17.0%,and a temperature at which the viscosity thereof grows to 109 poise ranges 650 to 680° C. and a temperature at which the viscosity thereof grows to 1012 ranges 555 to 585° C., and the difference between these temperatures is in the range of 96 to 103° C.
A glass composition for manufacturing a transparent sheet glass disclosed in the PCT 8-500811 comprises;    69-75 wt. % SiO2;    0-3 wt. % Al2O3;    2-10 wt. % CaO;    0-2 wt. % MgO;    9-17 wt. % Na2O;    0-8 wt. % K2O; and
0.2-1.5 wt. %Fe2O3, and furthermore comprises fluorine; oxides of zinc, zirconium, cerium and titanium; less than 4 wt. % barium oxide; and other oxides of alkaline earth metals wherein a total amount of the oxides of alkaline earth metals other than barium oxide is 10 wt. % or less.
Recently, a variety of glasses having an ultraviolet/infrared absorptivity and a greenish color shade to be used as a vehicle windshield have been proposed with the view of preventing degradation of luxurious interior materials and reducing the load of air conditioning of the vehicle.
For example, Japanese patent publication H3-187946A discloses a glass which has an ultraviolet transmittance of 38% or less, a solar energy transmittance of 46% or less, and a visible light transmittance of at least 70% in order to secure a field of vision from inside of a vehicle. As a color shade of these glasses for vehicles with greenish color shade, a greenish color shade tinged blue tends to be preferred.
It is known that the solar energy transmittance of a glass decreases when the amount of FeO in the total iron oxide which is added to the glass increases. Almost all of the infrared absorbent glasses which were proposed in the past use this method.
The aforementioned glass composition disclosed in JP H6-53592B contains a large amount of Al2O3, particularly equal to or more than 3% Al2O3 from a total amount of Al2O3 and TiO2 specified as above. In addition, to produce clear glass which is not yellow very much, the glass composition needs to contain a large amount of Al2O3 without containing TiO2 and unfavorably becomes hard to melt. In an embodiment of the JP H6-53592, a sample of the reinforced glass having a thickness of 3 mm is shown, wherein the surface compressive stress is not enough in spite of its improved condition of reinforcement.
The composition of the glass to be reinforced easily disclosed in aforementioned JP H4-60059B is capable of providing a reinforced glass which is relatively easy to be reinforced in such a manner to control viscosity relative to temperature. However, the temperature difference between the temperatures giving the viscosity of 109 poise and the viscosity of 1012 poise is as small as 7° C. Therefore the glass composition is so narrow that the glass is hard to be produced.
In a glass composition for manufacturing transparent sheet glass which is disclosed in the PCT (Japanese phase) H8-500811, a total amount of oxides of alkaline earth metals is not greater than 10 wt. % to obtain permeability. However, the amount of oxides of alkaline metals is needed to be increased to keep viscosity, which reduces durability. Furthermore, particularly, in case of reinforcing a glass plate having a thickness of 3.1 mm or less, a sufficient compressive stress value can not be obtained.